1. Field of the Invention
This invention relates to method and apparatus for producing a wound electrode assembly by overlaying and winding electrode strips and separator strips.
2. Description of the Related Art
An example of the method for producing a wound electrode assembly is described in Japanese Patent Application Publication No. 2007-142351 (JP-A-2007-142351). An apparatus disclosed in this publication is arranged to take up elements while pressing a pressure roller (corresponding to a pressing member) against elements (substantially corresponding to electrode strips and separator strips) that are being wound, so as to make the outside diameter and hardness of the wound elements stable or uniform. In this apparatus, while the elements are wound into a roll, the pressure roller is pressed against the elements toward a core of the roll, so as to discharge air trapped in clearances between the elements. However, there is no specific suggestion in JP-A-2007-142351 about how to press the pressure roller against the elements, in particular, how to adjust the pressing force of the pressure roller.
Another conventional example is described in Japanese Patent Application Publication No. 10-310299 (JP-A-10-310299). This publication discloses a method for stably controlling tension of strips (substantially corresponding to electrode strips and separator strips) to be wound up, using a dancer mechanism.
In the meantime, the inventor of the present invention contemplates a method for producing the wound electrode assembly, in which the winding speed is increased so as to improve the production efficiency. For example, the inventor contemplates taking up strip materials having an overall length of about 5 m at a winding speed that exceeds about 2 m/s, so that winding time (i.e., a length of time required to wind up the strip materials) can be significantly reduced. In this case, the winding speed is sharply increased from the beginning of winding, and then sharply reduced before the end of winding. Thus, the winding speed is increased and reduced at a much higher rate than before.
Various problems arise if the winding speed is increased or reduced at such a high rate. For example, if the winding speed is sharply increased from the beginning of winding, the tension applied to the electrode strips and separator strips increases sharply. Also, if the winding speed is sharply reduced before the end of winding, the tension decreases sharply. If the tension decreases sharply, air is likely to be trapped in the wound strip materials, and the wound electrode assembly becomes soft. On the other hand, if the pressing member (pressure roller) is strongly pressed against the electrode strips and separator strips while they are wound, so as to prevent air from being trapped in the wound strip materials, the resulting wound electrode assembly becomes harder than necessary.